1. Field of the Invention
The present invention relates to a color cathode ray tube(CRT), and more particularly, to an electron gun for a color CRT for generating electron beams.
2. Background of the Related Art
FIG. 1 illustrates a section of a first exemplary related art color CRT with an electron gun. In general, the CRT is a display for displaying a desired image by directing three electron beams 2 to a fluorescent film 3 on an inside surface of a panel 1, to make the fluorescent film 3 luminescent. The CRT is provided with an electron gun 4 for emitting the electron beams.
The first exemplary related art electron gun is provided with three independent cathodes 40, and first, second, third, fourth, fifth, and sixth electrodes 41, 42, 43, 44, 45, and 46 disposed at intervals in a tube axis direction. There is a shield cup 47 fitted to a screen side of the sixth electrode 46. Upon application of power to stem pins 5 on the electron gun 4, heaters heat the cathodes 40 causing them to emit electrons. The amount of emitted electrons is controlled by the first electrode 41, accelerated by the second electrode 42, and converged and accelerated by a pre-focus lens formed between the third, fourth, and fifth electrodes 43, 44, and 45. Then, the electron beams 2 are precisely focused onto a preset scan position by a main lens having a strong focusing power formed by a potential difference between the fifth electrode 45 and the sixth electrode 46.
The diameter of the main lens fixes a spot size of the electron beam 2. That is, if the main lens has a smaller diameter with a greater spherical aberration, the spot size of the electron beams passing through the main lens becomes greater, and if the main lens has a greater diameter with a smaller spherical aberration, a spot size of the electron beams passing through the main lens becomes smaller, relatively. The diameter of the main lens is dependent on electron beam pass through apertures formed on opposing sides of the fifth and sixth electrodes 45 and 46. If the size of the electron beam pass through apertures is large, the diameter of the main lens is the larger, and opposite to this, if the size of the electron beam pass through aperture is small, the diameter of the main lens is comparatively smaller.
Therefore, in the electron gun in the first exemplary related art CRT, the three electron beam pass through apertures (not shown) formed in opposing surfaces of the fifth and sixth electrodes 45 and 46 form a main lens proportional to the size of electron beam pass through apertures. Though the size of the electron beam pass through apertures should be formed greater for forming a greater main lens, the size is limited. Accordingly, the electron gun in the first exemplary related art CRT, which has a small main lens diameter, is mostly used in a small sized Braun tube or a Braun tube, requiring a low resolution.
FIG. 2 illustrates a perspective view of key parts of a second exemplary related art electron gun, showing the fifth and sixth electrodes provided for enlarging the main lens. (See U.S. Pat. No. 4,406,970). The second exemplary related art electron gun is a modified version of the first exemplary related art electron gun for enlarging the main lens. That is, the second exemplary related art electron gun is provided with rim portions 45b and 46b of race track forms on opposing surfaces of the fifth electrode 45 (or focus electrode) and the sixth electrode 46 (or anode electrode), recesses with recess surfaces 45c and 46c inside of the rim portions 45b and 46b, and three electron beam pass through apertures 45a, 46a in each of the recess surfaces 45c and 46c. Since the recesses 45d and 46d formed in the opposing surfaces of the electrodes act as apertures, a main lens proportional to the recesses can be obtained between the fifth electrode 45 and the sixth electrode 46, permitting use of a main lens that is comparatively greater than the first exemplary electron gun.
FIG. 3 illustrates a perspective view of key parts of a third exemplary related art electron gun, showing the fifth and sixth electrodes provided for enlarging the main lens. (See U.S. Pat. No. 4,599,534).
Referring to FIG. 3, in order to enlarge the diameter of the main lens, the third exemplary related art electron gun is provided with rim portions 45b and 46b of race track forms in opposing surfaces of the fifth electrode 45 and the sixth electrode 46 for common pass through of the three electron beams, and field control electrodes 45e and 46e of plates approx. 0.6xcx9c0.7 mm thick and fixed at locations recessed at a depth from the rim portions 45b and 46b for forming identical lens power for the electron beams 2. Each of the electric field control electrodes 45e and 46e has vertically elongated electron beam pass through apertures 45a and 46a at their centers, which have a horizontal diameter that is shorter than a vertical diameter, and half cut away vertically elongated apertures adjacent to the apertures 45a and 46a at the centers for pass through of outer electron beams. There is also a correction electrode 48 of an angle form fitted to the shield cup 47 electrically connected to the sixth electrode 46. End portions of the rim portions 45b and 46b are curved inwardly by approx. 1 mm. Thus, also in the third exemplary related art electron gun, inside portions of the rim portions 45b and 46b formed in opposing surfaces of the fifth electrode 45 and the sixth electrode 46 serve as apertures, to provide a large diameter main lens. In comparison to the second exemplary related art electron gun, the third exemplary related art electron gun can provide a uniform lens action to the three electron beams by means of the electric field control electrodes 45e and 46e formed in the fifth and sixth electrodes 45 and 46, and can correct the vertical direction strong lens action into a horizontal direction by means of the vertically elongated rim portions 45b and 46b. 
FIG. 4 illustrates a perspective view of key parts of a fourth exemplary related art electron gun, similar to the second and the third examples provided for enlarging the main lens.
Referring to FIG. 4, the fourth exemplary related art electron gun is provided with rim portions 45b and 46b of race track forms in opposing surfaces of the fifth electrode 45 and the sixth electrode 46 for pass through of the three electron beams in common, with the insides of the rim portions fully opened, and electric field control electrodes 45e and 46e having rectangular electron beam pass through apertures 45a and 46a curved inwardly for the center electron beam at locations inside of opened portions 45f and 46f. Each of the rim portions 45b and 46b are curved inwardly by 1 mm for reinforcing the electrodes for preventing distortion of the diameter during fabrication.
FIG. 5 illustrates a section of a fifth exemplary related art electron gun. The fifth exemplary related art electron gun 4 is an electron gun for forming a dynamic quadrupole lens (xe2x80x9cDQxe2x80x9d) having a lens action against a deflection yoke. That is, the fifth electrode, a focus electrode, is divided into a 5-1 electrode 50 and a 5-2 electrode 51, and the DQ lens is provided between the 5-1 electrode 50 and the 5-2 electrode 51. The DQ lens corrects a vertical elongation of a spot of an electron beam in forming a circular spot. This electron gun is used in a Braun tube requiring a high resolution or a large sized Braun tube to prevent distortion of an image along the periphery of a screen.
FIG. 6 illustrates a perspective view of key parts of a sixth exemplary related art electron gun. The sixth exemplary related art electron gun also utilizes a DQ lens, wherein the fifth electrode is divided into a 5-1 electrode 50 and a 5-2 electrode 52, three vertically elongated electron beam pass through apertures 50a of key aperture forms are formed in a surface of the 5-1 electrode 50 opposing a corresponding site of the 5-2 electrode 52, an electric field control electrode 51 of a plate form having three circular electron beam pass through apertures is provided inside of the 5-1 electrode 50, three vertically elongated electron beam pass through apertures 52a of key aperture forms are formed in a surface of the 5-2 electrode 52 opposite to the 5-1 electrode 50, and a plate projection 53 projects from both the top and bottom of each of the electron beam pass through apertures 52a in a horizontal direction. The plate projection 53 on the center aperture has a height h1 that is comparatively higher than a height h2 of the plate projection 53 on the outer apertures.
Electron gun design parameters, that influence spot diameter include a magnifying power of the lens, a spatial charge repulsive force, and a spherical aberration of the main lens. However, the influence of the lens magnifying power to the spot diameter Dx is of little use as a design parameter since the voltage distance of focus, and length of the electron gun are basically fixed. In order to reduce enlargement of the spot diameter xe2x80x9cDstxe2x80x9d coming from the spatial charge repulsive force, a phenomenon in which electrons in the electron beams repel and collide with one another to enlarge the spot diameter, it is favorable that an angle of advance of the electron beam (divergence angle xe2x80x9cxcex1xe2x80x9d) is designed to be enlarged. Opposite to spatial charge repulsive force, the spherical aberration of the main lens, denoting that an enlarged spot diameter xe2x80x9cDicxe2x80x9d caused by a difference in focus of electrons passed through a radical axis of the lens and a protaxis of the lens, forms a smaller spot diameter as the divergence angle of the electron beam incident to the main lens becomes smaller. In general, the spot diameter xe2x80x9cDtxe2x80x9d on a screen can be expressed with an equation, below.
Dt={square root over ((Dx+Dst+L )2+Dic2+L )},
where, Dx represents a spot diameter by a lens magnifying power, Dst represents a spot diameter by the spatial charge repulsive force, and Dic represents a spot diameter by a difference of electrons passed through a radical axis and a protaxis, i.e., a spherical aberration. Particularly, the best method to reduce the spatial charge repulsive force and the spherical aberration at the same time is to enlarge the diameter of the main lens, to reduce spot enlargement caused by spherical aberration even if the electron beam has a great divergence angle, and to reduce the spatial charge repulsive force after the electron beam passes through the main lens.
FIGS. 7 and 8 illustrate a drawing and a graph showing a method for calculating a main lens diameter. After calculating an optimal objective distance for a fixed voltage, configuration, and focal distance, an electron beam is passed through the main lens. A graph is then plotted taking the divergence angle xcex1 and a diameter R of the electron beam as axes as shown in FIG. 9, so that a lens diameter of a particular main lens is calculated into an equivalent diameter of a circular main lens by comparison with the circular lens. FIG. 8 illustrates that a horizontal main lens diameter H is equivalent to approx. 11.5 mm, and a vertical main lens diameter V is equivalent to approx. 7.6 mm. FIG. 9 illustrates a main lens diameter vs. a spot diameter, wherein it can be known that increasing the diameter of the main lens while decreasing the spherical aberration of the main lens reduces the spot diameter. The main lens diameter can be enlarged by enlarging a physical aperture diameter of the main lens, or by designing a depth of the electric field control electrode which corrects the lens greater. However, physical enlargement of the electrode aperture diameter is limited in that a diameter of the neck portion is limited to 29.1 mm. Accordingly, a design in which a position of the electrostatic field control electrode for the fifth and sixth electrodes, main lens forming electrodes, is placed deeper has been made. However, when a depth L1 from a surface of the fifth electrode 45 opposite to the sixth electrode 46 to the electrostatic field control electrode is approx. 3 mm and a depth L2 from a surface of the sixth electrode 46 opposite to the fifth electrode 45 to the electrostatic field control electrode is greater than approx. 3.6 mm, it is impossible provide a design which satisfies the same convergences of lenses and desired astigmatisms for the three electron beams and out of beam convergence characteristics (xe2x80x9cOCVxe2x80x9d), which is a distance between outer beams on the screen caused by convergence of the outer beams toward a central beam. Therefore, the maximum main lens diameter obtainable from the main lens is 8.8 mm in a horizontal direction and 7.8 mm in a vertical direction as shown in TABLE 1.
For improvement of focus to keep pace with the requirements for high resolution images and employment of a high frequency, a reduction of a horizontal spot diameter on the screen is keenly required, which in turn requires an increased main lens diameter. And, of the exemplary related art electron guns, the fifth and sixth exemplary related art electron guns show the horizontal direction diameter of the center lens being smaller by approx. 0.7 mm than the outer lenses. Therefore, in order to obtain an optimal DQ lens action, strengthening of the DQ lens action of the outer lenses is required because aspect ratios of the outer electron beams in the main lens portion after the electron beams passed through the DQ lens are higher than that of the center beam. That is, as shown in FIG. 11, in order to increase the DQ lens action, the height of the plate projection 53 on the center aperture in the 5-2 electrode 52 in the DQ lens portion should be higher than the height of the plate projection 53 on the outer apertures since the height of the plate projection on the 5-1 electrode 51 should be higher in the fifth electrode, for satisfying a horizontal direction focusing power for the spot in a periphery of the screen.
Accordingly, the present invention is directed to an electron gun for a color CRT that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an electron gun for a color CRT which can improve focus characteristics.
Another object of the present invention is to provide an electron gun for a color CRT, which can correct inconsistency between a center beam and outer beams in a DQ lens that occurs when a spot diameter on a screen is reduced by enlarging a main lens diameter.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the electron gun for a color CRT includes a plurality of cathodes each for emitting an electron beam, a triode unit having a control electrode and an accelerating electrode for controlling an amount of emission of the electron beam, a pre-focus lens unit having at least two electrodes for converging the electron beam, two electrodes for forming a main lens for focusing the electron beam onto a screen, each of the two electrodes for forming a main lens having a rim portion in an opposite surface of respective main lens forming electrodes common for the three electron beams, and an electrostatic field control electrode positioned a distance from the rim portion, wherein, in the main lens forming electrodes, a depth from the rim portion of an anode to the electrostatic field control electrode is deeper than the depth from the rim portion of a cathode to the electrostatic field control electrode.
In another aspect of the present invention, there is provided an electron gun for a color CRT including a plurality of cathodes each for emitting an electron beam, a triode unit having a control electrode and an accelerating electrode for controlling an amount of emission of the electron beam, a pre-focus lens unit having at least two electrodes for converging the electron beam, two electrodes for forming a main lens for focusing the electron beam onto a screen, each of the two electrodes for forming a main lens having a cup electrode with a rim portion in an opposite surface of respective main lens forming electrode common for the three electron beams, an electrostatic field control electrode positioned a distance inside from the rim portion having three electron beam pass through apertures, and an electrode of a cap form, three of them being connected electrically, wherein, in the main lens forming electrodes, relations between a maximum horizontal diameter H, a maximum vertical diameter V, and a distance from the rim portion to the electrostatic field control electrode L for at least one of the electrodes can be expressed as follows.
Lxe2x89xa74.0(V/H)+2.1.
In other aspect of the present invention, there is provided an electron gun for a color CRT including a plurality of cathodes each for emitting an electron beam, a triode unit having a control electrode and an accelerating electrode for controlling an amount of emission of the electron beam, a pre-focus lens unit having at least two electrodes for converging the electron beam, a DQ lens unit having at least two electrodes for removing a vertical halo from all region of a screen, two electrodes for forming a main lens for focusing the electron beam onto the screen, the main lens has a relation expressed as follows, a diameter of the main lensxe2x89xa7(neck diameterxc3x970.26)+1.4, wherein a DQ lens action of a center beam portion formed by the DQ lens unit is weaker than the DQ lens action of an outer electron beam portion.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.